Versatile Interplay of Chalcogenide and Dichalcogenide Anions in the Thiovanadate Ba7S(VS3O)2(S2)3 and Its Selenide Derivatives: Elaboration and DFT Meta-GGA Study

Oxychalcogenides are emerging as promising alternative candidates for a variety of applications including for energy. Only few phases among them show the presence of Q–Q bonds (Q = chalcogenide anion) while they drastically alter the electronic structure and allow further structural flexibility. Four original oxy(poly)chalcogenide compounds in the system Ba–V–Q–O (Q = S, Se) were synthesized, characterized, and studied using density functional theory (DFT). The new structure type found for Ba₇V₂O₂S₁₃, which can be written as Ba₇S(VS₃O)₂(S₂)₃, was substituted to yield three selenide derivatives Ba₇V₂O₂S_9.304Se_3.696, Ba₇V₂O₂S_7.15Se_5.85, and Ba₇V₂O₂S_6.85Se_6.15. They represent original multiple-anion lattices and first members in the system Ba–V–Se–S–O. They exhibit in the first layer heteroleptic tetrahedra V⁵⁺S₃O and isolated Q^2– anions and in the second layer dichalcogenide pairs (Q₂)^2– with Q = S or Se. Selenide derivatives were attempted by targeting the selective substitution of isolated Q^2– or (Q₂)^2– (in distinct layers) or both by selenide, but it systematically led to concomitant and partial substitution of both sites. A DFT meta-GGA study showed that selective substitution yields local constraints due to rigid VO₃S and pairs. Experimentally, incorporation of selenide in both layers avoids geometrical mismatch and constraints. In such systems, we show that the interplay between the O/S anionic ratio around V⁵⁺, together with the presence/nature of the dichalcogenides (Q₂)^2– and isolated Q^2–, impacts in unique manners the band gap and provides a rich background to tune the band gap and the symmetry.